Contamination of the air by toxic gases, especially ammonia gas, is a common problem in both agricultural and industrial environments. For example, personnel engaged in the production of blueprints from drawings are exposed to heavy concentrations of ammonia, as generally are those persons engaged in the production or manufacture of fertilizers.
In those situations where animals or poultry are grown or kept in confined quarters, the anaerobic decomposition of their own fecal matter results in a constant exposure of the poultry to concentrations of ammonia that can seriously impair their health, there being a large catalog of ailments attributable to such concentrations. In a copending U.S. Pat. application, Ser. No. 202,557 of D. Ball, filed June 6, 1988, there is disclosed a method and apparatus for monitoring the air within the confined animal quarters for the presence of ammonia and for effectively neutralizing the ammonia in response to detection of concentrations above a threshold quantitative level. The ammonia is generally neutralized by the introduction of a carboxylic acid such as acetic or formic acid into the air.
In general, there are two principal types of ammonia sensors in use, metal oxide semiconductor (MOS) and electrochemical. The MOS sensor generally consists of a heated cylinder of tin oxide doped with an appropriate doping agent, the electrical resistance of which decreases in the presence of ammonia or other organic combustible gases. Such MOS sensors do not discriminate effectively among the several such gases, and they can be affected by changes in temperature and humidity. On the other hand, such sensors are relatively low cost, virtually indestructible and have extended lifetimes, e.g. five to ten years.
Electrochemical ammonia sensors usually consist of a pair of electrodes, a gas permeable membrane, and an electrolyte that is selectively responsive to ammonia. Such sensors generally are not responsive to other organic gases and are unaffected by temperature and humidity changes. On the other hand, they have lifetimes measured in months instead of years, and are considerably more expensive than MOS sensors.